Is there any way to get -12 V (DC) using only:
- +12 V DC
- Ground
- OPAMPs
- Resistors
- Capacitors
- Inductors
- Diodes
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1\$\begingroup\$ is this a homework problem? \$\endgroup\$– markragesCommented Dec 10, 2010 at 21:29
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\$\begingroup\$ More of a project that has some insane limitations. \$\endgroup\$– user2188Commented Dec 10, 2010 at 21:41
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\$\begingroup\$ @Brian: Homework tag was agreed on meta not to be used because it does not specify the contents. I used projects in that purpose. \$\endgroup\$– user2188Commented Dec 10, 2010 at 21:46
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\$\begingroup\$ DC, Homework, Projects are all way too broad \$\endgroup\$– Nick TCommented Dec 10, 2010 at 22:13
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3 Answers
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- Label the wire which is currently ground "-12V".
- Label the wire which was previously 12V "GND". (There is no step 3)
To test: Connect a DMM's ground lead to the wire you've labeled "GND". Connect the positive lead to the wire you've labeled "-12V". The display will read -12 volts.
Of course, this won't work if you've got any other signals referenced to the original ground, so it's rather tongue-in-cheek. Just wanted to point it out in case it wasn't obvious.
More seriously, my solution would be a charge pump driven by an RC oscillator. Without transistors, this will limit you to the output current of your opamps. On further consideration, it might not be possible due to the problem of bootstrapping your opamp power supplies.
answered Dec 10, 2010 at 22:07
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\$\begingroup\$ I actually did think about that today(well, the reverse where i dub 12V as 24V, GND as 12V and -12 as GND), and was surprised to see you come up with this answer too. I believe that my circuit will behave well with this configuration as it will not require that much modifying, thanks anyway. \$\endgroup\$– user2188Commented Dec 12, 2010 at 1:01
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\$\begingroup\$ @tyblu - I was, but I'm not sure if Cem is.... \$\endgroup\$ Commented Dec 12, 2010 at 20:40
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\$\begingroup\$ @reemrevniek, heh - sorry, was not directed at your answer; I like it! \$\endgroup\$– tybluCommented Dec 12, 2010 at 21:33
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Of course, you can make nearly anything with all of those. Depending on your current/power requirements, you are looking at one of the following switching supplies:
- charge pump : a personal favourite, requiring only a cap and uC pin
- boost, with its many variants
Normally one would use a dedicated SMPS controller, with internal oscillator, and gate driver along with a handful of transistors to implement these, but apparently you can't use them. You must design a harmonic oscillator out of inductors and capacitors, then buffer and clean up (make square) this signal with op amps. With this chopper signal you can implement either of the above SMPSs as well as stabilize the LC oscillator (it will die).
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LM7660 or equivalent *7660 part.
Have a look how it works, it is fairly easy to implement with an op-amp and a few external components.
I'm not going to do all your homework, though.
answered Dec 10, 2010 at 21:33